Local mail-specific:
1. * When downloading pop3 mail, leave an x-mozilla-keys header with space for writing tags.2. * When parsing a mailbox, parse the x-mozilla-keys header(s)3. * When setting tags on a local message, write to x-mozilla-keys header if room4. * When compacting a folder, if we have more tags in the db for the msg than we have space in the mailbox, add an other x-mozilla-key header5. * Make sure x-mozilla-keys are stripped the same way x-account-key is stripped
Tag <-> Keyword mapping service
